Can Membership own their listing
I just started using the Membership plugin about 2 weeks ago Larry, so my answer is based on limited experience...
As you are setting up the membership plugin, you can choose for the membership database to be a separate data table than your CMSB users, or you can choose for them to be the same table.
If you want your users to be able to update their own listings, you would want the Membership and CMSB data tables to be THE SAME.
Then on the user-signup.php file you would include a line so that every person signing up has Author Access to your listings table. They would then be able to login to the CMSB backend using the same username/password as the membership. They would only see the link to the Listings table, and even there they would only be able to see and edit their own listings.